If you piss on an electric fence, that won't send the current up the stream. I saw an exhibit about this some years back at the Museum of Science. THey put a stream of water in a strobe-lit chamber and showed how it is not really a solid stream of water but lots of little droplets that are so close together your eyes can't distinguish where one ends and the others begin. And even if it was a solid stream, you wouldn't get the electrons to travel up it because once the urine hits the surface the molecules and ions fall down and don't get back up.
So piss on the electric fence as much as you want. you won't hurt anything.